Electric spindle motor

ABSTRACT

An electric spindle motor has a base assembly (19) and a hollow notatable hub (16) housing a motor. The hub (16) is mounted for rotation about an axis and a stator (12) is mounted on the base assembly (19) inside the hub (16). A liquid filled journal bearing (11, 21) acts between the base assembly (19) and the hub (16) to provide radial support of the hub (16). A gas filled thrust bearing (26, 27, 35) also acts between the hub (16) and base assembly (19) to provide at least axial support of the hub (16).

This application is a continuation of Ser. No. 08/887,738 filed Jul. 3,1997.

TECHNICAL FIELD

This invention relates to electric spindle motors of the kind used formagnetic disk drives. Although the invention will hereinafter bedescribed with reference to a spindle motor for hard disk drives, thespindle motor of this invention is also suitable for use in otherapplications where high speed precision spinning motion is required.

BACKGROUND ART

Disk drives are a commonly used data storage device which utilisemagnetic medium. It is required that the disk drive spindles have a lowmagnitude of random vibration in both the axial and radial directions.In addition, the basic requirements for practical magnetic recordingdevices are: high stiffness, especially in the radial direction, highshock resistance, and being capable of operating normally despite theorientation of the spindle axis. As consumer demands for the computerproducts, in particular personal computers, continuously push theadvances of the magnetic recording technology towards miniaturisation,high storage capacity, and fast data transfer rate, the shortcomings ofthe conventional ball bearing supported spindle assembly becomes moreand more acute. Major problems with the usage of a ball bearing spindleassembly are its high level of non-repeatable-runout (NRR), and wearrate. Because of the non-uniformity and geometric imperfectness ofbearing balls, inner and outer races, unpredictable runout can occurduring operation. This represents the main constrains for the datastorage track width which has to accommodate the magnitude of theirregular vibration in the radial and the axial directions. As a result,the maximum achievable track density is limited primarily by the levelof NRR. In practice, pre-loading of the ball bearing system of thespindle is used to reduce the NRR. However, excessive pre-loading forcecauses further increase in the wear rate and frictional losses, whilstany further miniaturisation of the disk drives necessitates lower powerlosses since heat dissipation becomes more difficult.

Another shortcoming of ball bearing spindle motors is that a sealmechanism is required to prevent any wear debris, dust, foreign bodiesand evaporated substances from exhaling out of the bearings andcontaminating the magnetic data storage media. The performance of thisseal mechanism tends to degrade with increased wear and it is thereforenot suitable for high speed applications. A further limitation ariseswith prior art ball bearing systems because scalable reduction of thesize of the ball bearing structures is subject to unavoidablelimitations and cannot always fit in the progressively miniaturised diskdrives.

Fluid film bearings (FFB) appear to be a promising alternative tosatisfy the demands for high precision spindles suitable for highcapacity magnetic recording systems. In a self-acting FFB system, thebearing surfaces are kept separated by a lubricant film. That is, thereis no metal to metal rubbing during operation, and therefore acomparatively low wear rate. The most outstanding feature of thisbearing structure, however, is that it provides extremely low NRRspinning, as compared with the ball bearing spindle. The spindle motorstructure described in U.S. Pat. No. 4,200,344 to Binns is one of theearliest inventions which makes use of the FFB technology. Like manyothers, for example, that described in U.S. Pat. No. 4,656,545 toKakuta, it operates unidirectional, which limits the commercialapplications as modern disk drives are required to operate regardlessthe shaft orientation.

One particular problem with the application of the cylindrical FFBjournal bearing system is that it may lose its stability duringoperation. This is because the radial stiffness of the FFB journalbearing is low when the radial load is small, for example, when thespindle axis is vertically oriented. A particular consequence isinstability due to sub-rotating-frequency whirl. As an economic andeffective approach to achieve an enhanced bearing stability, tapered orconical bearings can be used. U.S. Pat. No. 4,734,606 to Hajec discloseselectric motor constructions with tapered FFB lubricated by ferrofluid.Tapered air-lubricated FFB spindles for magnetic recording devices aredescribed in the U.S. Pat. No. 5,283,491 to Jabbar.

With the application of liquid-lubricated FFB spindles, there comes theproblem of sealing of the fluid. The bearing lubricant must be securelyconfined by a seal mechanism since leakage of lubricant into the spacereceiving the data storage disks causes contamination of the recordingmedia and thus malfunction of the disk drives. Leakage of the lubricantcan also cause degradation of the bearing performance, resulting infailure of the disk read and write processes of disk drive systems. Theseal may also cause excess frictional loss, for example, if a contactseal is used. The seal must additionally be designed to withstand a highlevel of shock. Other design options using sophisticated sealingmechanisms may only be realised at a high manufacturing cost. An exampleof such a hard drive spindle is disclosed in U.S. Pat. No. 5,246,294 toPan.

Gas (including air) lubricated bearing spindles which avoid thenecessity to seal the lubricant are attractive. In another spindle unitdescribed in the U.S. Pat. No. 5,127,744 to White et al., a part of thestationary shaft is fitted with a rotationary ceramic sleeve to form thejournal bearing, whilst two disc-shaped ceramic thrust bearings aredisposed at both ends of the bearing sleeve. A bearing structure of thiskind, that is, two thrust hearings disposed at two ends of a journalbearing, occupies a significant portion of the space in the spindle hub.It may be suitable for use in the "underslung" topology for the spindlemotor, in which the motor components are placed below the spindle hub.

In practice, gas lubricated bearing spindles suffer from the majorweakness of difficulty in achieving sufficient stiffness. In thisrespect, the stiffness of the journal bearing is of far more concernsince the required radial stiffness is not only critical but also,taking the magnetic disk drives by way of an example, must be higherthan the required axial stiffness in many cases. Measures to enhance therigidity of the air bearing system, for example, to reduce the bearingclearance, will inevitably increase the manufacturing cost. Furthermore,practical disk drives must be designed to withstand a high level ofshock. When the rotating mass is large, for instance for high capacitydisk drives which have a large number of disks, the stiffness constraintbecomes more stringent. As a result, the utilisation of the self-actingFFB system lubricated fully by gas has been limited to cases where thepassive load is relatively light, for example, the spindle of thepolygon mirror scanners.

DISCLOSURES OF THE INVENTION

The present invention thus attempts to overcome one or more of theproblems associated with the prior art ball and fluid bearing supportedspindles as described above, and provide a spindle motor structure thatis more suitable for magnetic disk drives with enhanced systemperformance, particularly for high load, high speed applications.

Accordingly, in one aspect this invention provides an electric spindlemotor comprising a base assembly, a hollow rotatable hub housing amotor, said hub being mounted for co-joined rotation with a centralshaft about a rotational axis, a stator mounted with said base assemblyand disposed within said hub a liquid filled journal bearing actingbetween said shaft and said base assembly to provide radial support ofsaid hub, and a gas filled thrust bearing acting between said hub andsaid base assembly to provide at least axial support of said hub.

Preferably, the base includes a cylindrical sleeve which extendscoaxially of the shaft and houses the journal bearing. The journalbearing is preferably formed between the exterior surface of the shaftand the interior surface of the sleeve. A dynamic pressure generatinggroove pattern is preferably formed on either or both of the surfacesforming the bearing. The liquid lubricant used in the bearing preferablyhas a viscosity of about 4 cSt at 100° C. A non-contact surface tensionliquid seal is preferably used to contain the liquid within the journalbearing. The seal is preferably formed by an outwardly tapered portionof the interior surface of the cylindrical sleeve. An antimigrationcoating is also preferably applied adjacent the seal.

The thrust bearing preferably includes a thrust plate having a thrustsurface extending outwardly from the rotational axis and acorrespondingly shaped first bearing surface spaced apart from the firstthrust surface by first bearing clearance. Preferably, the thrust plateis a double sided tapered bearing, which also provides a second thrustsurface extending outwardly from the rotational axis and acorrespondingly shaped second bearing surface. Preferably, the thrustsurfaces and/or bearing surfaces are provided with groove patterns toimprove bearing performance. An air passage is preferably provided toallow smooth circulation of air flow to the bearing and preventstagnation. A seating configuration is preferably provided using wearresistant material to protect the bearing surfaces during starting andstopping of the spindle motor.

The spindle motor of this invention is able to achieve an extremely lowlevel of non-repeatable runout and thereby increase the recordingdensity of an associated disk drive. Additionally, the bearing systemshave a relatively low wear rate and a very low level of noise andvibration compared with conventional ball bearing systems. The bearingsystem can provide sufficient radial and axial stiffness for the spindlein any orientation of the spindle axis. The effective seal mechanism ofthe preferred embodiment ensures that the bearing system is free fromthe risk of lubricant leakage.

It will also be apparent that the tapered thrust bearing of thepreferred embodiment provides axial load capability for the spindlemotor and also partially supports the radial load. The extra stiffnessin the radial direction provided by tapered thrust bearing increases thestability of the spindle particularly when the radial load of thespindle is very small, for example, when the spindle axis is verticallyoriented.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 is a cross-sectional view of an embodiment according to thepresent invention.

FIG. 2 is a cross-sectional view of an embodiment of the prior art diskdrive spindle.

FIG. 3(a) is a detailed cross-sectional view of the air lubricatedthrust bearing.

FIG. 3(b) is the groove patterns on upper and lower surfaces of thrustplate with pumping-in grooves on one surface and pumping-out grooves onthe other.

FIG. 3(c) is the groove patterns on the upper and lower surfaces of thethrust plate with pumping-in effect grooves on both surfaces.

FIG. 4 is a cross-sectional view of grooved journal bearing with itstapered seal.

FIG. 5 is an alternative thrust bearing design for the embodiment of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

For better understanding of the basic principle on which the disk drivespindle operates, a conventional spindle structure is first described.FIG. 2 shows cross-sectional view of a prior art disk drive spindlewhich employs ball bearing technology. The basic configuration of thisspindle motor assembly is known as "in-spindle motor", in which themotor components are located completely inside the spindle hub. Thistype of spindle is typically used for disk drives having a relativelylarger number of disk platters, eg: 4 platters or more. As illustratedin FIG. 2 the common motor configuration for disk drives includes abrushless dc motor, having an outer rotor with permanent magnet poles,resides inside the hub. The spindle motor unit comprises a cylindricalsteel shaft 11 which is fitted with a stator 12 consisting of thelamination core 14 and armature windings 13. A rotatable hub 16,preferably made of aluminum alloys, has a cylindrical hole runningthrough it to receive the ball bearing 22 at the top end, and the sleeve25 which in turn supports the bearing 23 at the bottom end of thespindle. Inside the hub 16 and between the ball bearings 22 and 23,there is a cavity where the stator resides. An air gap 15 is formedbetween the outer surface of the stator core 14 and the inner surface ofthe ring-shaped magnetic poles 18 which is attached to the rotor backiron 17, typically made of ferromagnetic material such as low carbonsteel is to provide the magnetic path for the electric motor. Theterminal leads of the armature windings 13 are led out of the cavity ofthe hub 16 through the passage way 24 in the centre of the shaft 11 andare connected to the power supply of the disk drive via a motor controlcircuit of known kind (not shown). The hub 16 may also have a magneticflux ring or the commutator 49. The signals regarding to the position ofthe rotating hub assembly is produced by the commutator during operationand picked up by Hall sensors feeding to the control circuit. However,the commutator will be unnecessary if a sensorless drive circuit is usedfor the spindle unit. When the armature winding 13 is energisedaccording to the control logic used to regulate the speed of thespindle, an electromagnetic torque is developed by the interactionbetween magnetic fields due to the magnet poles and the armaturecurrents, respectively, and causes rotation movement of the spindle.

Referring to FIG. 1, a cross-section view of an embodiment of thepresent invention is illustrated. The basic elements of the electricmotor and the principle of its operation for the spindle assembly shownin FIG. 2 are essentially the same as the conventional ball bearingspindle, shown in FIG. 2. The bearing system of the preferred embodimentcomprises an air-lubricated, conical thrust bearing with spiral grooves,and a liquid-lubricated journal bearing with herringbone grooves and asurface tension seal to confine the lubricant.

As shown in FIG. 1, a cylindrical shaft 11 is fitted to the rotatablehub 16. The shaft has a journal bearing portion 42. A mating bearingsurface 43 is formed on the inner surface of a stationary cylindricalsleeve extending from a base plate 19. Shaft 11 has slightly smallerdiameter than the inner diameter in the journal bearing portion ofsleeve 21, thus forming a bearing clearance 44 which is typically in arange of 2-15 micrometres depending primarily on the actual load of thespindle system. The journal bearing will be described in detail belowwith reference to FIG. 4. The bottom end of the sleeve 21 is fitted onto the base plate 19 which has a through hole 20 for the terminal leadsof the armature windings. A thrust bearing plate 35 is mounted coaxiallyonto the other end of the sleeve. Also the stator 12 of the electricmotor is mounted onto the stationary sleeve in the cavity of thecylindrically hollow hub 16. The rotor of the electric motor includingmagnetic poles 18 and back iron 17, is fitted to the hub 16 and facesthe stator core 14.

The spindle unit of FIG. 1 further comprises a pair of identical,tapered thrust bearing sleeve members 26 and 27 attached to the innersurface of the hub 16. The inner surfaces 33 and 34 of the taperedsleeve member 26 and 27 are the mating surfaces for the thrust bearingplate 35 having bearing surfaces 38 and 39, as shown in FIG. 3(a), whichillustrates the details of the air lubricated thrust bearing. Insidedimensions of the upper and lower thrust bearing members 26 and 27 areslightly larger than the outside dimensions of the tapered thrust plate35. This results in gaps 36 and 37 for the thrust bearing. The taperedsurfaces 33, 34, 38 and 39 of the respective surfaces of the doublesided thrust bearing make an angle of 12.5° with respect to thehorizontal axis. Groove patterns are provided at the surfaces 38 and 39of the thrust plate 35 in order to enhance the hydrodynamic pressuresgenerated during operation. The spiral groove patterns at the surfaces38 and 39 can be designed to produce inward-pumping effects for the airflow in the clearances 36 and 37 as shown in FIG. 3(b). Alternativelythey can be designed to generate inward-pumping effect for one side ofthe thrust bearing and outward-pumping effect for the other as shown inFIG. 3(c). In the present embodiment, the surfaces 31 and 32 of thetapered sleeves for the thrust bearing are coated with anti-wearmaterial, eg: silicon nitride (Si₃ N₄) silicon carbide (SiC), alumina(AL₂ O₃) or the like, to provide protection for the bearing surfacesduring starting/stopping processes. As shown in FIG. 1, a passage way 28is made at the top end of the hub 16. Together with the passage ways 29,30 and 40, this provides a smooth circulation of air flow for doublesided thrust bearing.

Referring now to FIG. 4, the two surfaces of the journal bearing areseparated by a film of liquid lubricant 50. A hydrodynamic pressure willbe generated in this journal bearing during operation to support thejournal load. Herringbone grooves are provided for this portion of theshaft, in this particular embodiment, to enhance the performance of thebearing, particularly the stiffness. In the spindle assembly, a surfacetension dynamic seal or otherwise known as capillary seal is used toconfine the lubricant within the active portion of the journal bearing.The surface tension sealing is realised by the tapered surfaces 45 and46 formed at the inner surface of the sleeve 21 near two ends of thejournal bearing, as shown in FIG. 1. Since it is a non-contact type ofseal, the associated power loss is very low.

The operation of the spindle motor will be described for two typicaldispositions, i.e. when the spindle axis is vertical and horizontal.

When the spindle axis is vertically oriented as shown in FIG. 1, at restposition, the rotating system sits on surface 51 of the thrust platewith surface 31 of the thrust bearing sleeve. A wear resistant materialsuch as described above is coated on the surface 31 by physical orchemical vapor deposition to reduce the wear during starting andstopping. In the start-up process, the load due to the rotating memberis initially supported on surface 51. As the motor speeds up, surface 31loses contact with its sliding surface 51, and the weight of therotating hub assembly, which includes weight of hub 16, the shaft 11,the tapered sleeves for the thrust bearing and the disk platters, issupported by the hydrodynamic pressure developed in gap 37. Ahydrodynamic pressure is also developed during operation in the gap 36between the bearing surface 33 of the upper thrust bearing 27 and thesurface 38 of the thrust plate. This hydrodynamic pressure together withthe total load of the rotating hub assembly and the hydrodynamicpressure developed in the gap 37 between bearing surfaces 34 and 39,defines the working film thickness and therefore the position of thethrust plate within the upper and lower thrust bearings. In addition tosupporting the load in the axial direction, the hydrodynamic pressurealso has radial component which can contribute to the radial stiffnessof the spindle when the shaft is vertically-oriented and the radial loadis very small. When the spindle is placed in the up-side down positionwith the base plate facing upwards, the wear resistance surface 31provides the sitting for the rotatable hub assembly, and duringoperation the weight will be supported by the hydrodynamic pressuredeveloped in the gap 36 between the surface 33 of the thrust bearing 27and the surface 38 of the thrust plate 35. In FIG. 3(a), the passage way40 for air circulation is also illustrated.

When the spindle axis is horizontally oriented, the weight of the wholerotating hub assembly is supported by the hydrodynamic pressuredeveloped in the journal bearing which has herringbone grooves, and thehydrodynamic pressure developed in the double sided thrust bearing. Atthis position, the conical thrust bearing can provide axial stiffnessfor the spindle unit.

FIG. 5 is a cross-sectional view of an alternative embodiment of thepresent invention. In this spindle structure the thrust bearing plate 35is designed with flat surfaces at both ends having hydrodynamic pressuregenerating grooves. With this spindle structure, the thrust bearing doesnot produce extra stiffness in the zero load direction, i.e. the radialdirection when the spindle axis is vertically oriented, or the axialdirection when the spindle axis is horizontally oriented. However, themanufacturing complexity and therefore costs for a flat thrust bearingmember with groove patterns are much lower than those for a conicalthrust bearing.

Although the present invention has been described with reference to apreferred embodiment, it will be apparent to those skilled in the artthat changes and modifications may be made in form and detail withoutdeparting from the spirit and scope of the invention.

We claim:
 1. An electric spindle motor, comprising:a base plate; astationary sleeve extending substantially perpendicular from said baseplate along a rotational axis; a shaft extending within said sleevealong said rotational axis and spaced apart therefrom to define a firstclearance gap; a liquid fluid situated within said first clearance gapfor providing at least radial stiffness for said shaft; a rotatablehousing situated in a manner that forms a cavity between said housingand said base plate, said shaft and said sleeve being situated withinsaid cavity; a stator situated within said cavity and coaxially aroundsaid shaft and stationary sleeve; a plate situated within said cavityand extending radially outward from said stationary sleeve; a rotatablesleeve situated within said cavity and extending radially inward fromsaid housing, said rotating sleeve including upper and lower surfacesthat are complementary shaped with upper and lower surfaces of saidplate and spaced apart therefrom to form a second clearance gap; and agas fluid situated within said second clearance gap for providing atleast axial stiffness for said shaft.
 2. The electric spindle motor ofclaim 1, wherein the surface of said shaft is grooved for increasing thehydrodynamic pressure of the liquid fluid for improved radial stiffnessof said shaft.
 3. The electric spindle motor of claim 2, wherein thegrooves on the surface of said shaft are arranged in a herring bonepattern.
 4. The electric spindle motor of claim 1, wherein the surfaceof said stationary sleeve is grooved for increasing the hydrodynamicpressure of the liquid fluid for improved radial stiffness of saidshaft.
 5. The electric spindle motor of claim 2, wherein the grooves onthe surface of said stationary sleeve are arranged in a herring bonepattern.
 6. The electric spindle motor of claim 1, wherein said firstclearance gap is between 4 and 20 micrometers.
 7. The electric spindlemotor of claim 1, further including a non-contact surface tension liquidseal for preventing leakage of said liquid fluid.
 8. The electricspindle motor of claim 7, wherein said seal is formed by an outwardlydivergent portion of the interior of said stationary sleeve near an endof said stationary sleeve.
 9. The electric spindle motor of claim 8,wherein said outwardly divergent portion is a conical taper.
 10. Theelectric spindle motor of claim 7, wherein said non-contact seal isaided by an antimigration coating over an inner surface of saidstationary sleeve.
 11. The electric spindle motor of claim 1, wherein atleast upper or lower surfaces of said plate and said rotatable sleeveare inclined with respect to a normal to the rotational axis.
 12. Theelectric spindle motor of claim 11, wherein both upper and lowersurfaces of said plate and said rotatable sleeve are oppositely inclinedon either side of the normal to the rotational axis.
 13. The electricspindle motor of claim 1, wherein at least one of said upper and lowersurfaces of said plate and said rotatable sleeve include a groovepattern to provide gas flow through said second clearance gap and toprovide axial stiffness of said shaft.
 14. The electric spindle motor ofclaim 13, wherein said groove pattern includes a spiral and/or a herringbone pattern.
 15. The electric spindle motor of claim 1, wherein saidplate is tapered to have a reducing cross section in the radiallyoutward direction.
 16. The electric spindle motor of claim 1, whereinsaid upper and lower surfaces of said plate and/or said upper and lowersurfaces of said rotatable sleeve include a groove pattern for causinggas flow through said second clearance gap.
 17. The electric spindlemotor of claim 16, wherein said groove pattern includes a spiral and/ora herring bone pattern.
 18. The electric spindle motor of claim 16,wherein said groove pattern is arranged to produce an inward gas flowwithin said second clearance gap.
 19. The electric spindle motor ofclaim 16, wherein said groove pattern is arranged to produce an inwardgas flow within said second clearance gap situated between respectiveupper surfaces of said plate and said rotatable sleeve and outward gasflow within said second clearance gap between respective lower surfacesof said plate and said rotatable sleeve.
 20. The electric spindle motorof claim 16, wherein said groove pattern is arranged to produce aninward gas flow within said second clearance gap situated betweenrespective lower surfaces of said plate and said rotatable sleeve andoutward gas flow within said second clearance gap between respectiveupper surfaces of said plate and said rotatable sleeve.
 21. The electricspindle motor of claim 16, wherein said rotatable sleeve includespassageways for providing circulation of gas within said secondclearance gap.
 22. The electric spindle motor of claim 1, wherein saidplate is attached to said stationary sleeve.
 23. The electric spindlemotor of claim 1, wherein said stationary sleeve is attached to saidbase plate.
 24. The electric spindle motor of claim 1, wherein saidplate is annular and surrounds said stationary sleeve.
 25. The electricspindle motor of claim 1, wherein said gas fluid is air.
 26. Theelectric spindle motor of claim 1, wherein said housing includes anopening for the admission of gas fluid into said second clearance gap.27. The electric spindle motor of claim 1, wherein at least one surfaceof said plate and/or said rotatable sleeve includes a layer of wearresistance material for protecting said one surface.
 28. The electricspindle motor of claim 1, wherein said first clearance gap is between 2and 15 micrometers.
 29. The electric spindle motor of claim 1, whereinsaid second clearance gap is between 2 and 15 micrometers.
 30. Theelectric spindle motor of claim 1, wherein said rotable housing iscoupled to said shaft for rotation therewith.
 31. An electric motor,comprising:a first sleeve; a shaft extending within said first sleevealong a rotational axis and spaced apart therefrom to define a firstclearance gap; a liquid fluid situated within said first clearance gapfor providing at least radial stiffness for said shaft; a statorsituated coaxially around said first sleeve and shaft; a stationaryplate having upper and lower surfaces; a second sleeve including agroove that is complementary shaped with upper and lower surfaces ofsaid plate and spaced apart therefrom to form a second clearance gap;and a gas fluid situated within said second clearance gap for providingat least axial stiffness for said shaft.
 32. A spindle motorcomprising:a stationary base; a shaft extended from the said base; afirst rotatable sleeve extending with said shaft along a rotational axisand spaced apart therefrom to define a first clearance gap; a liquidlubricant filled in said first clearance gap for providing at leastradial stiffness for said first rotatable sleeve, the said lubricant isconfined by a non-contact sealing; a housing coupled to said rotatablesleeve for rotation therewith and in a manner that forms a cavitybetween said housing and said base plate, said shaft and sleeve beingsituated within said cavity; a stator attached to said base and situatedwithin said cavity coaxially around said shaft; a thrust having upperand lower surfaces and extending radially outwards from said shaft; asecond rotatable sleeve situated within said cavity and extendingradially inward from said housing, said rotating sleeve including upperand lower inner surfaces that are complementary shaped with upper andlower surfaces of said plate and spaced apart therefrom to form a secondclearance gap, and a gas fluid situated within said second clearance gapfor providing at least axial stiffness for said shaft.